The present invention relates to improvements in a method for obtaining a patterned photoresist layer of which the resist pattern has a reduced dimension by a post-patterning heat treatment. More particularly, the invention relates to improvements in the method for reducing the pattern dimension in a photolithographically patterned resist layer by a post-patterning procedure in which a patterned resist layer on a substrate is provided thereon with a coating layer of a water-soluble resin and then the thus coated patterned resist layer is subjected to a heat treatment to effect thermal shrinkage of the resist layer resulting in a reduced pattern dimension followed by complete removal of the coating layer of the water-soluble resin by washing with water.
Along with the recent trend in the technology of semiconductor devices toward higher and higher degrees of integration and more and more compact sizes of the devices, the photolithographic patterning technology of photoresist layers is also required to accomplish finer and finer patterning of the photoresist layer.
An approach for accomplishing the above mentioned requirement in the photolithographic technology for a pattern dimension of 0.20 xcexcm or finer is to use a patterning exposure light of very short wavelengths such as KrF excimer laser beams, ArF excimer laser beams and F2 excimer laser beams as well as electron beams. This approach, however, cannot be successful without development of a photoresist composition having adaptability to these short-wavelength exposure radiations.
In this regard, so-called chemical-amplification photoresist compositions are widely employed in the modern photolithographic technology, in which the catalytic activity of an acid generated in the light-exposed areas from a radiation-sensitive acid-generating agent contained in the resist layer is utilized to effect a solubility change of the resinous ingredient to give high sensitivity and pattern resolution even with a small amount of the acid-generating agent.
As a method for obtaining very finely patterned resist layer on a substrate, there is a known method in which a photoresist layer formed on a substrate is patterned in a conventional way including patterning light-exposure and development and the thus patterned resist layer is provided with a coating layer of a resin by utilizing the activity of the acid diffused from the resist layer followed by a heat treatment so as to effect reduction of the pattern dimension to be finer than the resolution limit inherent in the photoresist composition (Japanese Patent Kokai 5-166717 and 5-241348).
This method, however, has a problem in respect of the relatively large temperature dependency amounting to more than 10 nm/xc2x0 C. within the substrate surface. This disadvantage can hardly be overcome with the heating device currently employed in the manufacture of semiconductor devices due to poor uniformity of the temperature distribution. Accordingly, the above-described method of post-patterning dimension-reducing method cannot be practiced without substantial variations in the pattern dimensions.
It is also known that the dimension of a patterned resist layer can be reduced below the resolution limit of the photoresist composition by subjecting the patterned resist layer to a heat treatment or a radiation irradiation treatment to cause mobilization of the patterned resist layer. Though advantageous in respect of small temperature dependency of only a few nm/xc2x0 C. within the plane of the substrate surface, this method has a problem that, due to the difficulty in controlling mobilization of the photoresist layer by the heat treatment, uniform reduction of the dimension of the photoresist layer can hardly be expected within the substrate surface.
Besides the above described photolithographic patterning process by utilizing excimer laser beams for patterning light exposure, proposals are made in Japanese Patent No. 2723260 for reduction of the pattern dimension, according to which a layer of an electron beam resist composition comprising a polymethyl methacrylate resin is patterned to give a patterned resist layer which is then provided thereon with a coating layer of a positive-working resist composition followed by a heat treatment to form a reacted layer at the interface between the patterned resist layer and the positive-working resist layer and removal of the positive-working resist layer from the unreacted areas. Japanese Patent Kokai 6-250379 further discloses a method in which a reacted layer is formed between the underlying patterned resist layer and the upper resist layer by utilizing the acid generated from the acid-generating agent or thermal crosslinking by the acid. Japanese Patent Kokai 10-73927 discloses a method for the manufacture of semiconductor devices by effecting reduction of the pattern dimension in which the overcoating layer is formed by using, as the coating solution, a composition prepared by dissolving a water-soluble resin, water-soluble crosslinking agent or combination thereof in a water-miscible solvent without addition of any photosensitive ingredients. Japanese Patent Kokai 2000-347414 proposes a method in which a substrate surface is provided thereon with a photosensitive layer of a chemical-amplification photoresist composition which is subjected to patterning light-exposure and development to form a patterned resist layer, a coating film is formed on the patterned resist layer by using a coating composition containing a water-soluble resin such as a polyvinyl acetal, water-soluble crosslinking agent such as tetra(hydroxymethyl)glycoluril, a water-soluble nitrogen-containing compound such as amines and, optionally, a fluorine- and silicon-containing surface active agent followed by a heat treatment to form a water-insoluble reacted layer at the interface between the patterned resist layer and the overcoating layer and finally the overcoating layer in the unreacted areas is removed by using a solvent.
Although each of the above-described methods is desirable because reduction of the pattern dimension can be conveniently accomplished to exceed the wavelength limitation of the photoresist composition by forming an upper coating layer on the underlying photoresist layer, several disadvantages are involved therein. For example, the crosslinking reaction of the overcoating composition may overly proceed to unnecessary portions such as the bottom of the patterned resist layer resulting in an undesirable non-orthogonal cross sectional profile thereof eventually with trailing skirts. The dimension of the upper resist layer depends on the mixing baking which is a heat treatment to cause crosslinking. Further, the temperature dependency obtained by these methods is relatively large to be 10 nm/xc2x0 C. or larger so that it is very difficult to ensure high uniformity of the pattern dimension within the substrate surface when the substrate has a large size or the patterned resist layer is extremely fine resulting in poor controllability in reduction of the pattern dimension.
Besides, a proposal is made in Japanese Patent Kokai 1-307228 and 4-364021 for the so-called thermal flow process in which a patterned photoresist layer formed on a substrate is subjected to a heat treatment or radiation-irradiation treatment to effect mobilization of the resist layer so as to accomplish reduction of the pattern dimension to become finer than the resolution limit of the photoresist composition.
This method, however, is defective because products of reproducible quality can hardly be obtained due to the difficulty encountered in controlling the mobility of the resist by means of heat or radiation. As a further development of this thermal flow process, Japanese Patent Kokai 7-45510 proposes a method in which the mobility of the resist is controlled by providing a coating layer of a water-soluble resin on the patterned photoresist layer formed on a substrate. Since the water-soluble resin used in this method, such as polyvinyl alcohols, is insufficient in the solubility in water required in the removal with water and long-term stability, troubles are sometimes caused by the residual resin film remaining unremoved with water.
In view of the above described problems and disadvantages in the prior art, the present invention has an object to provide an improvement in the method for reducing the dimension of a patterned photoresist layer by a post-patterning treatment in which the patterned resist layer is provided thereon with a coating layer of a water-soluble resin composition followed by a heat treatment to cause reduction of the pattern dimension or the distance between the resist patterns and then removal of the water-soluble coating layer away from the patterned resist layer by washing with water.
Thus, in a first aspect of the invention, the present invention provides, in a method for reducing a pattern dimension in a patterned photoresist layer formed on a substrate by a post-patterning heat treatment, referred to as the coated thermal flow process hereinafter, comprising the steps of: (a) forming a coating layer of a water-soluble resin composition on the patterned resist layer, (b) drying the coating layer of the aqueous coating solution, (c) subjecting the dried coating layer and the patterned resist layer to a heat treatment to effect thermal shrinkage of the patterned resist layer with reduction of the pattern dimension and (d) removing the coating layer of the water-soluble resin composition, the improvement which comprises: using, in step (a), a coating solution containing a water-soluble resin and a water-soluble amine compound having, preferably, a pKa value of 7.5 to 13.0 at 25xc2x0 C. for the formation of the coating layer.
In a second aspect of the invention, the improvement provided by the present invention comprises, in step (a) of the coated thermal flow process, using an aqueous coating solution containing a water-soluble resin which is a copolymer of (A) acrylic acid, methacrylic acid or a combination thereof and (B) a water-soluble ethylenically unsaturated compound which is exemplified by N-vinylpyrrolidone, N-vinylimidazolidinone, methyl acrylate, methyl methacrylate, N,N-dimethylacrylamide, N,N-dimethylaminopropyl methacrylamide, N,N-dimethylaminopropyl acrylamide, N-methhylacrylamide, diacetoneacrylamide, N,N-dimethylaminoethyl methacrylate, N,N-diethylaminoethyl methacrylate, N,N-dimethylaminoethyl acrylate, N-acryloylmorpholine and the like, of which those nitrogen-containing water-soluble compounds are preferable, or a combination thereof.
In a third aspect of the invention, the present invention is directed to an improvement of the aqueous coating solution for use in step (a) of the coated thermal flow process in which the water-soluble resin is a copolymer of N-vinylpyrrolidone and a water-soluble monomeric vinyl compound other than N-vinylpyrrolidone which is preferably N-vinylimidazolidinone.
In a fourth aspect of the invention, the present invention provides, in the above-mentioned coated thermal flow process of a patterned resist layer, the improvement which comprises, as a guideline for the selection of the water-soluble resin in step (a) of the method using an aqueous coating solution containing a water-soluble resin exhibiting such a water-solubility behavior that, in a testing procedure comprising the steps of forming a coating layer of the resin on an unpatterned photocured layer of the photoresist composition, subjecting the coating layer to a heat treatment at 140xc2x0 C. for 60 seconds and removing away the coating film by washing with water at 23xc2x0 C., the coating layer can be completely removed within 60 seconds.